The present invention relates to a method and to an apparatus for cutting by electrical discharges an electrode workpiece by means of an electrode tool in the form of a wire, in the course of which electrical discharges are applied across the electrodes and the electrodes are fed relative to each other according to a programmed cutting path, the electrode wire being longitudinally displaced through the machining zone by being subjected to a predetermined pull between two guide members.
As is the case in all EDM machines, the speed of feed of the electrode tool into the workpiece is adjusted such as to maintain a predetermined electrical discharge gap between the electrode wire and the workpiece. The machine performances with respect to precision and speed of machining are limited by the mechanical and thermal loads which can safely be applied to the electrode wire. The pulling force applied on the wire influences its position between the wire guide members and consequently the accuracy of machining, more particularly during changes in the direction of the cutting path and in the optimum cutting speed, and the permissible pulling force is closely related to the intensity of the current which may flow through the wire without causing rupture of the wire. It is therefore important to control with accuracy the physical state of the wire at all times, such as to apply through the wire a current of an intensity compatible with the force of traction exerted on the wire, or vice versa.
It is known, for example, to adjust the traction exerted on the wire as a function of the elongation of the wire under traction, but such a method is not very precise and does not take into consideration the heat expansion of the wire.
The principal object of the invention is to utilize a characteristic magnitude representative of the physical state of the wire which permits to control both the mechanical and thermal stress of the wire, such as to obtain an automatic regulation of the machining current or of the force exerting a pull on the wire during machining, or of both. The characteristic magnitude is the ohmic resistance of the wire which varies both as a function of the machining current and as a function of the traction exerted on the wire. It has been observed that the limit of the force of traction causing rupture of the wire varies as a function of the current intensity, and that this limit causing rupture of the wire corresponds to values of the wire electrical resistance which are within a very narrow range. Maintaining the electrical resistance of the wire below a predetermined value permits to avoid rupture of the wire.